The present disclosure relates to a passive optical network (PON), more particularly to a PON maintenance method, and discloses a method for maintaining the PON, an optical network unit (ONU), and an optical line terminal (OLT).
In the ever growing broadband access networks, most existing local access networks (LANs) run at 100 Megabits per second (Mbps) and many large businesses are transitioning to Gigabit Ethernet (GE). The metropolitan core networks (MCNs) and the metropolitan edge networks (MENs) have abundant synchronous optical network (SONET), synchronous digital hierarchy (SDH), and GE bandwidth capacity, which means that the access network is a serious bandwidth bottleneck. Compared to cable transmission, the optical fiber transmission has a number of advantages including large capacity, low loss, and high immunity to electromagnetic interference. Therefore, fiber access networks are the inevitable development trend as the cost of optical fiber transmission gradually decreases. The last mile access networks require low cost, simple structure, and easy-to-implement capabilities, which is a challenge to achieve technologically. As one of the key fiber access technologies, the passive optical network (PON) implements fiber access to various depths. Based on the depth of the fiber's reach, the fiber access can be divided into fiber to the home (FTTH), fiber to the building (FTTB), fiber to the curb (FTTCurb), fiber to the cabinet (FTTC), and fiber to the premises (FTTP), which are collectively referred to as fiber-to-X (FTTX).
Based on the content it carries, the PON technologies are divided into Asynchronous Transfer Mode Based PONs (APONs), Ethernet Based PONs (EPONs), and Gigabit PONs (GPONs). As shown in FIG. 1, the PON includes an optical line terminal (OLT) residing at the central office and a series of optical network units (ONUs) or optical network terminals (ONTs) residing at the Customer Premises Network. The OLT and the ONU/ONT are connected together by the optical distribution network (ODN), which includes optical fiber, passive optical dividers, and/or optical couplers.
In a PON, a single optical fiber cable can be laid from the service exchange to the broadband service area or office area. The main optical fiber may be divided into a number of branches that go to individual buildings or service terminals using a passive optical divider or coupler. Such an approach will enable multiple users to share the relatively expensive optical link from the exchange to the Customer Premises Network, thus significantly reducing the cost of the FTTB and the FTTH.
There are no active components in the PON from the central exchange to the Customer Premises Network because they are replaced by passive optical components. The point-to-multipoint transmission is achieved by power distribution over the entire path. The replacement eliminates the need for the service provider to maintain and supply power to the active components, thus dramatically cutting the cost to the service provider. The passive optical divider and coupler serve as a light transmitting and limiting means, and do not require a power supply or data processing. The passive optical divider and coupler also have unlimited mean time between failures (MTBF), thus reducing the overall maintenance cost for the service provider.
Through the use of APON, BPON, EPON, or the soon to be standardized GPON technology, the backbone optical fiber cable in the PON can support 155 Mbps, 622 Mbps, 1.25 Gigabits per second (Gbps), or 2.5 Gbps. To support voice, data, and video applications simultaneously, the bandwidth allocated to each user can be static or dynamic.
Where the OLT has a basic architecture as shown in FIG. 2 and the ONU/ONT has a basic architecture as shown in FIG. 3, both the OLT and the ONU/ONT include an optical module (OM) for receiving and/or transmitting optical signals, a service processing module (SPM), and a power supply module. The OM comprises:
a receiving circuit connected to the upstream channel for receiving the upstream channel signal and for optical-electrical conversion;
a transmitting circuit connected to downstream channel for converting a received electrical signal into an optical signal and then sending the optical signal via the downstream channel;
a signal detection (SD) circuit for monitoring whether the upstream channel is continuously seized, e.g. whether there is an incoming optical signal, and outputting the monitoring result via the SD wiring terminal; wherein when there is an incoming optical signal, the SD wiring terminal outputs a high level signal; otherwise, a low level signal is output; and
an operation power supply circuit for supplying power to the OM, wherein the power supplied for the transmitting and receiving circuits are combined in the existing technologies.
The SPM end of the OLT connects to the upstream channel network interface at the office end via a central network interface (CNI), and the ONU/ONT end of the OLT connects to the user equipment via a user network interface (UNI). Of course, the basic architecture of the OLT and the ONU/ONT may also include a control module.
The data traffic going downstream in the PON is broadcasted from the OLT to each ONU/ONT, and each ONU/ONT transfers the address information in the cell header through a matching protocol and processes only the data that matches the ONU/ONT's address. The transmission of traffic going upstream is relatively complex. In a point-to-multipoint PON system, due to the medium-sharing characteristic of ODN, all of the ONU/ONT terminals transmit data to the OLT using time division multiple access (TDMA). To avoid conflicts, normally the OLT will allocate different timeslots (authorization) to the registered ONU/ONTs according to the data buffer (to be transmitted) reported by the ONU/ONTs. Each ONU turns on its OM only during its allocated timeslot. Therefore, in normal operations, the upstream channel is used on a timeslot basis, and the SD signal from the OLT's OM is a pulsed signal.
The OLT assigns a timeslot (authorization) to each ONU to ensure that only one ONU is emitting light at a given time, e.g. a particular transmission timeslot represents a particular ONU/ONT so that conflicts between the different ONU/ONT bursts can be avoided by synchronizing these timeslots. However, the OM of a particular ONU/ONT may fail and enter a constant light emitting (CLE) state, or the OM may be set by a malicious user to the CLE state, which can be easily achieved by changing the polarity of the OM transmission enabling pin. When the OM suffers such a failure, then all of the other ONUs that are connected to the same OLT port as the faulty ONU will also fail. Such a result may be unacceptable compared to the traditional point-to-point system.